Blender Internal How to slow down and crop movie textures?
You might consider an alternative - load the video into Blender's video editor and producing a copy, reducing it's framerate and cropping it at the same time.  Use the new video instead for the texture.  

I'm still using Blender Vers 2.77a and find when reducing a video's speed in the VSE (editor) it's best not to use the editor's 'Speed Control' effects strip.  It's output is jerky-jerky.

Instead, go to the render properties panel at right in the 3D window and set the frame rate to half or whatever rate you need.

You can if you wish, select "Custom" for the frame rate and use the 'Divide-by' slot that's below the frame rate entry slot.  It's marked with a "/" divide symbol.  Enter a 2 in there for half speed, 4 for quarter and so on.

The results are reasonably smooth when you consider that a half frame rate is only 15 or 12 fps.



Render the video and use that to texture the original plane or object.

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Cropping -

Whilst you're in the VSE you can also crop the new video.  

Cropping a video is only a matter of making sure the video strip (not the sound) is highlited.  

Press N and tick the "Image Crop" box at right and adjust the cropping sliders

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If you prefer to crop a textured plane in the 3D window, it's only a matter of placing another plane over the section you want to cut out and adjusting it's Material--> Transparency tab to .008.  Set both it's specular values to zero.



Make sure it's actually closer to the camera or viewpoint than the original plane.   This will render any part of the plane that's underneath, invisible.  See pic.

If you have trouble with this method, I can do a quick video.  It does have advantages over the VSE's and Compositor's cropping.
It's downside might be the need to render using "OpenGL", not the internal Blender render, but there is a way around that also by "filming" the cropped video and using that for the texture.